This invention relates to a novel method of forming a pattern on a substrate. In particular, it relates to the use of a high solids content solution of a partially imidized polyamic acid to coat and form a pattern on the substrate.
Because of their excellent thermal, insulation, and adhesion properties polyimides have been used as passivation, buffer or dielectric layer coatings in semiconductor fabrication. To open vias for connecting conductive wires underneath polyimide coatings, two patterning methods have been developed. In one method (see U.S. Pat. No. 4,369,090), a solution of a polyamic acid is prepared by dissolving a diamine and a dianhydride in an organic solvent. The solution is applied to the substrate and is heated to evaporate the solvent and partially imidize the polyamic acid. Partial imidization is necessary to reduce swelling of the coating during the patterning process and thereby form a better pattern by reducing shrinkage during imidization. However, the polyamic acid cannot be fully imidized on the substrate because the resulting polyimide would be insoluble and portions of it could not be dissolved to form a pattern. A photoresist is applied over the partially imidized polyamic acid coating and it is exposed to a pattern of light. Those portions of the photoresist that were exposed to the light (or that were not exposed to the light) are soluble in a solvent and are washed away along with the partially imidized polyamic acid underneath. The remaining boating is heated to fully imidize the polyamic acid.
In another method of forming patterns of a polyimide coating on a substrate (see U.S. Pat. No. 4,880,722) the solution of the polyamic acid is mixed with a photosensitizer. The solution is applied to the substrate and is heated to evaporate the solvent and partially imidize the polyamic acid. The coating is exposed to a pattern of UV light which renders those portions of the photosensitizer that were exposed to the light soluble in an aqueous base developer. The insoluble photosensitizer is hydrophobic and prevents the polyamic acid with which it is in contact from dissolving in an aqueous base developer. The soluble portions of the coating and the underlying partially imidized polyamic acid are washed away with an aqueous base developer. The remaining coating is heated to fully imidize the polyamic acid. While this method works well, the photosensitizer may decompose during the partial imidization step.
In both methods, it is difficult to obtain low shrinkage coatings due to the partial imidization of the polyamic acid. In some applications coatings with less shrinkage are needed for thick insulation or protection of the substrate.